This invention relates to a control method of controlling the quantity of fuel being supplied to an internal combustion engine immediately after cranking thereof, and more particularly to such a control method, which is adapted to set the quantity of fuel being supplied to the engine immediately after cranking thereof to appropriate values in response to changes in the engine temperature, to thereby achieve stable operation of the engine.
Among conventional fuel supply quantity control methods for internal combustion engines, it has been generally known as starting fuel supply control to control the fuel quantity being supplied to the engine to an appropriate value corresponding to the cooling water temperature of the engine representative of the engine temperature, at cranking of the engine so as to ensure positive and smooth starting of the engine, while it has also been known as basic fuel supply control to control the fuel quantity being supplied to the engine to a value set by multiplying and/or adding to a basic value of the fuel quantity dependent upon operating parameters of the engine such as engine rotational speed and intake pipe absolute pressure by correction coefficients and/or correction variables depending upon the engine cooling water temperature, throttle valve opening, exhaust gas ingredient (O.sub.2) concentration, etc., after the engine has left the cranking state.
In order to obtain smooth transition from the cranking operation of the engine under the starting fuel supply control to normal operation of same under the basic fuel supply control, to prevent engine stall after the cranking of the engine, and to improve driveability of the engine at acceleration immediately after the cranking of same, a fuel supply control method has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 59-46329, which comprises setting an initial value of a fuel increment which is applied immediately after the cranking of the engine, in response to a product of a value of a cooling water temperature-dependent fuel increasing coefficient KTW of which the value decreases as the engine coolant temperature representative of the engine temperature increases, and a value of an after-start fuel increasing coefficient KAST, subsequently decreasing the initial value of the fuel increment by a predetermined value upon generation of each pulse of a top-dead-center (TDC) signal, and supplying the engine with a quantity of fuel set by the use of the thus set fuel increment.
According to this proposed fuel supply control method, however, because the value of the fuel increment is decreased in a substantially linear manner, the fuel quantity supplied to the engine does not always assume values appropriate for operating conditions of the engine.
To increase the fuel supply quantity after the cranking of the engine while the engine is in a cold state has originally been intended to compensate for leaning of the air/fuel mixture actually supplied to the engine due to incomplete evaporation of the fuel adhering to the cold inner walls of the intake pipe and the cylinders of the engine. However, the temperature of the inner walls of the cylinders rapidly increases as combustion repeatedly takes place within the same cylinder after the cranking of the engine, to promote the evaporation of fuel adhering to the inner walls of the cylinders, etc. Therefore, according to the above-mentioned fuel supply control method which decreases the fuel quantity in a substantially linear manner even while the temperature of the cylinders is rapidly increasing, the air/fuel mixture supplied to the engine becomes rich, deteriorating the ignition plug. To be specific, while during cranking, the air/fuel ratio of the mixture supplied to the engine should be very rich or smaller than 10 so as to make up for the adhesion of fuel to the inner walls or a small evaporation rate thereof, as described hereinabove, continued supply of such a rich mixture to the engine can cause accumulation of carbon on the plug or moistening of the plug with the fuel, adversely affecting the operation of the plug.
On the other hand, in order to ensure stable warming-up operation of the engine following the cranking, it is desirable to gradually decrease the fuel supply quantity so that the air/fuel ratio of the mixture becomes slightly richer than the theoretical air/fuel ratio, insofar as the aforesaid phenomena such as the accumulation of carbon on the plug can be avoided.
This requirement could be fulfilled, for instance, by accurately detecting the temperature of the inner walls of the cylinders and thereby setting the fuel supply quantity to a suitable value. However, in practice, the engine temperature is generally detected in terms of the engine cooling water temperature, which involves a problem of a time lag between a change in the temperature of the inner walls of the cylinders and the resulting change in the cooling water temperature, making it difficult to accurately detect the temperature of the inner walls of the cylinders.